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The connectivity of a neuron (its unique constellation of synaptic inputs and outputs) is essential for its function. Neuronal connections are made with exquisite accuracy between specific types of neurons. How each neuron finds its synaptic partners has been a central question in developmental neurobiology. We utilize the relatively simple nervous system of nematode C. elegans, to search for molecules that can specify synaptic connections and understand the molecular mechanisms of synaptic as

Projects

Mechanisms of Synaptic Specificity in C. elegans, National Institutes of Health

The major goals for this project are to understand how SYG-2 and SYG-1 interaction leads to the target choice of HSNL in C. elegans

Location

Stanford University, Department of Biology, Stanford, CA

Patterning dendritic branches with environmental and neuronal surface molecules, National Institutes of Health

The major goal of this project is to understand the molecular mechanisms underlying dendrite growth and branching

Publications

All Publications

Abstract

During synapse development, synaptic proteins must be targeted to sites of presynaptic release. Directed transport as well as local sequestration of synaptic vesicle precursors (SVPs), membranous organelles containing many synaptic proteins, might contribute to this process. Using neuron-wide time-lapse microscopy, we studied SVP dynamics in the DA9 motor neuron in Caenorhabditis elegans. SVP transport was highly dynamic and bi-directional throughout the entire neuron, including the dendrite. While SVP trafficking was anterogradely biased in axonal segments prior to the synaptic domain, directionality of SVP movement was stochastic in the dendrite and distal axon. Furthermore, frequency of movement and speed were variable between different compartments. These data provide evidence that SVP transport is differentially regulated in distinct neuronal domains. It also suggests that polarized SVP transport in concert with local vesicle capturing is necessary for accurate presynapse formation and maintenance. SVP trafficking analysis of two hypomorphs for UNC-104/KIF1A in combination with mathematical modeling identified directionality of movement, entry of SVPs into the axon as well as axonal speeds as the important determinants of steady-state SVP distributions. Furthermore, detailed dissection of speed distributions for wild-type and unc-104/kif1a mutant animals revealed an unexpected role for UNC-104/KIF1A in dendritic SVP trafficking.

Abstract

SYG-1 and SYG-2 are multipurpose cell adhesion molecules (CAMs) that have evolved across all major animal taxa to participate in diverse physiological functions, ranging from synapse formation to formation of the kidney filtration barrier. In the crystal structures of several SYG-1 and SYG-2 orthologs and their complexes, we find that SYG-1 orthologs homodimerize through a common, bispecific interface that similarly mediates an unusual orthogonal docking geometry in the heterophilic SYG-1/SYG-2 complex. C. elegans SYG-1's specification of proper synapse formation in vivo closely correlates with the heterophilic complex affinity, which appears to be tuned for optimal function. Furthermore, replacement of the interacting domains of SYG-1 and SYG-2 with those from CAM complexes that assume alternative docking geometries or the introduction of segmental flexibility compromised synaptic function. These results suggest that SYG extracellular complexes do not simply act as "molecular velcro" and that their distinct structural features are important in instructing synaptogenesis. PAPERFLICK:

Abstract

Axonal branching and synapse formation are tightly linked developmental events during the establishment of synaptic circuits. Newly formed synapses promote branch initiation and stability. However, little is known about molecular mechanisms that link these two processes. Here, we show that local assembly of an F-actin cytoskeleton at nascent presynaptic sites initiates both synapse formation and axon branching. We further find that assembly of the F-actin network requires a direct interaction between the synaptic cell adhesion molecule SYG-1 and a key regulator of actin cytoskeleton, the WVE-1/WAVE regulatory complex (WRC). SYG-1 cytoplasmic tail binds to the WRC using a consensus WRC interacting receptor sequence (WIRS). WRC mutants or mutating the SYG-1 WIRS motif leads to loss of local F-actin, synaptic material, and axonal branches. Together, these data suggest that synaptic adhesion molecules, which serve as a necessary component for both synaptogenesis and axonal branch formation, directly regulate subcellular actin cytoskeletal organization.

Abstract

In neuronal processes, microtubules (MTs) provide structural support and serve as tracks for molecular motors. While it is known that neuronal MTs are more stable than MTs in non-neuronal cells, the molecular mechanisms underlying this stability are not fully understood. In this study, we used live fluorescence microscopy to show that the C. elegans CAMSAP protein PTRN-1 localizes to puncta along neuronal processes, stabilizes MT foci, and promotes MT polymerization in neurites. Electron microscopy revealed that ptrn-1 null mutants have fewer MTs and abnormal MT organization in the PLM neuron. Animals grown with a MT depolymerizing drug caused synthetic defects in neurite branching in the absence of ptrn-1 function, indicating that PTRN-1 promotes MT stability. Further, ptrn-1 null mutants exhibited aberrant neurite morphology and synaptic vesicle localization that is partially dependent on dlk-1. Our results suggest that PTRN-1 represents an important mechanism for promoting MT stability in neurons. DOI: http://dx.doi.org/10.7554/eLife.01498.001.

Abstract

Gradients of topographic cues play essential roles in the organization of sensory systems by guiding axonal growth cones. Little is known about whether there are additional mechanisms for precise topographic mapping of synaptic connections. Whereas the C. elegans DA8 and DA9 neurons have similar axonal trajectories, their synapses are positioned in distinct but adjacent domains in the anterior-posterior axis. We found that two Wnts, LIN-44 and EGL-20, are responsible for this spatial organization of synapses. Both Wnts form putative posterior-high, anterior-low gradients. The posteriorly expressed LIN-44 inhibits synapse formation in both DA9 and DA8, and creates a synapse-free domain on both axons via LIN-17 /Frizzled. EGL-20, a more anteriorly expressed Wnt, inhibits synapse formation through MIG-1/Frizzled, which is expressed in DA8 but not in DA9. The Wnt-Frizzled specificity and selective Frizzled expression dictate the stereotyped, topographic positioning of synapses between these two neurons.

Abstract

Robust dendrite morphogenesis is a critical step in the development of reproducible neural circuits. However, little is known about the extracellular cues that pattern complex dendrite morphologies. In the model nematode Caenorhabditis elegans, the sensory neuron PVD establishes stereotypical, highly branched dendrite morphology. Here, we report the identification of a tripartite ligand-receptor complex of membrane adhesion molecules that is both necessary and sufficient to instruct spatially restricted growth and branching of PVD dendrites. The ligand complex SAX-7/L1CAM and MNR-1 function at defined locations in the surrounding hypodermal tissue, whereas DMA-1 acts as the cognate receptor on PVD. Mutations in this complex lead to dramatic defects in the formation, stabilization, and organization of the dendritic arbor. Ectopic expression of SAX-7 and MNR-1 generates a predictable, unnaturally patterned dendritic tree in a DMA-1-dependent manner. Both in vivo and in vitro experiments indicate that all three molecules are needed for interaction.

Abstract

SYD-2/liprin-? is a multi-domain protein that associates with and recruits multiple active zone molecules to form presynaptic specializations. Given SYD-2's critical role in synapse formation, its synaptogenic ability is likely tightly regulated. However, mechanisms that regulate SYD-2 function are poorly understood. In this study, we provide evidence that SYD-2's function may be regulated by interactions between its coiled-coil (CC) domains and sterile ?-motif (SAM) domains. We show that the N-terminal CC domains are necessary and sufficient to assemble functional synapses while C-terminal SAM domains are not, suggesting that the CC domains are responsible for the synaptogenic activity of SYD-2. Surprisingly, syd-2 alleles with single amino acid mutations in the SAM domain show strong loss of function phenotypes, suggesting that SAM domains also play an important role in SYD-2's function. A previously characterized syd-2 gain-of-function mutation within the CC domains is epistatic to the loss-of-function mutations in the SAM domain. In addition, yeast two-hybrid analysis showed interactions between the CC and SAM domains. Thus, the data is consistent with a model where the SAM domains regulate the CC domain-dependent synaptogenic activity of SYD-2. Taken together, our study provides new mechanistic insights into how SYD-2's activity may be modulated to regulate synapse formation during development.

Abstract

Cellular interactions between neighboring axons are essential for global topographic map formation. Here we show that axonal interactions also precisely instruct the location of synapses. Motoneurons form en passant synapses in Caenorhabditis elegans. Although axons from the same neuron class significantly overlap, each neuron innervates a unique and tiled segment of the muscle field by restricting its synapses to a distinct subaxonal domain-a phenomenon we term synaptic tiling. Using DA8 and DA9 motoneurons, we found that the synaptic tiling requires the PlexinA4 homolog, PLX-1, and two transmembrane semaphorins. In the plexin or semaphorin mutants, synaptic domains from both neurons expand and overlap with each other without guidance defects. In a semaphorin-dependent manner, PLX-1 is concentrated at the synapse-free axonal segment, delineating the tiling border. Furthermore, plexin inhibits presynapse formation by suppressing synaptic F-actin through its cytoplasmic GTPase-activating protein (GAP) domain. Hence, contact-dependent, intra-axonal plexin signaling specifies synaptic circuits by inhibiting synapse formation at the subcellular loci. VIDEO ABSTRACT:

Abstract

In neurons, microtubules (MTs) span the length of both axons and dendrites, and the molecular motors use these intracellular 'highways' to transport diverse cargo to the appropriate subcellular locations. Whereas axonal MTs are organized such that the plus-end is oriented out from the cell body, dendrites exhibit a mixed MTs polarity containing both minus-end-out and plus-end-out MTs. The molecular mechanisms underlying this differential organization, as well as its functional significance, are unknown. Here, we show that kinesin-1 is critical in establishing the characteristic minus-end-out MT organization of the dendrite in vivo. In unc-116 (kinesin-1/kinesin heavy chain) mutants, the dendritic MTs adopt an axonal-like plus-end-out organization. Kinesin-1 protein is able to cross-link anti-paralleled MTs in vitro. We propose that kinesin-1 regulates the dendrite MT polarity through directly gliding the plus-end-out MTs out of the dendrite using both the motor domain and the C-terminal MT-binding domain. DOI:http://dx.doi.org/10.7554/eLife.00133.001.

Abstract

The diverse cell types and the precise synaptic connectivity between them are the cardinal features of the nervous system. Little is known about how cell fate diversification is linked to synaptic target choices. Here we investigate how presynaptic neurons select one type of muscles, vm2, as a synaptic target and form synapses on its dendritic spine-like muscle arms. We found that the Notch-Delta pathway was required to distinguish target from non-target muscles. APX-1/Delta acts in surrounding cells including the non-target vm1 to activate LIN-12/Notch in the target vm2. LIN-12 functions cell-autonomously to up-regulate the expression of UNC-40/DCC and MADD-2 in vm2, which in turn function together to promote muscle arm formation and guidance. Ectopic expression of UNC-40/DCC in non-target vm1 muscle is sufficient to induce muscle arm extension from these cells. Therefore, the LIN-12/Notch signaling specifies target selection by selectively up-regulating guidance molecules and forming muscle arms in target cells. DOI:http://dx.doi.org/10.7554/eLife.00378.001.

Abstract

Wnt proteins play important roles in wiring neural circuits. Wnts regulate many aspects of neural circuit generation through their receptors and distinct signalling pathways. In this review, we discuss recent findings on the functions of Wnts in various aspects of neural circuit formation, including neuronal polarity, axon guidance, synapse formation, and synaptic plasticity in vertebrate and invertebrate nervous systems.

Abstract

We established a transcription-based binary gene expression system in Caenorhabditis elegans using the recently developed Q system. This system, derived from genes in Neurospora crassa, uses the transcriptional activator QF to induce the expression of target genes. Activation can be efficiently suppressed by the transcriptional repressor QS, and suppression can be relieved by the nontoxic small molecule quinic acid. We used QF, QS and quinic acid to achieve temporal and spatial control of transgene expression in various tissues in C. elegans. We also developed a split Q system, in which we separated QF into two parts encoding its DNA-binding and transcription-activation domains. Each domain showed negligible transcriptional activity when expressed alone, but expression of both reconstituted QF activity, providing additional combinatorial power to control gene expression.

Abstract

The sequestration of Muscleblind splicing regulators results in myotonic dystrophy. Previous work on Muscleblind has largely focused on its roles in muscle development and maintenance due to the skeletal and cardiac muscle degeneration phenotype observed in individuals with the disorder. However, a number of reported nervous system defects suggest that Muscleblind proteins function in other tissues as well.We have identified a mutation in the Caenorhabditis elegans homolog of Muscleblind, mbl-1, that is required for proper formation of neuromuscular junction (NMJ) synapses. mbl-1 mutants exhibit selective loss of the most distal NMJ synapses in a C. elegans motorneuron, DA9, visualized using the vesicle-associated protein RAB-3, as well as the active zone proteins SYD-2/liprin-? and UNC-10/Rim. The proximal NMJs appear to have normal pre- and postsynaptic specializations. Surprisingly, expressing a mbl-1 transgene in the presynaptic neuron is sufficient to rescue the synaptic defect, while muscle expression has no effect. Consistent with this result, mbl-1 is also expressed in neurons.Based on these results, we conclude that in addition to its functions in muscle, the Muscleblind splice regulators also function in neurons to regulate synapse formation.

Abstract

During synaptogenesis, macromolecular protein complexes assemble at the pre- and postsynaptic membrane. Extensive literature identifies many transmembrane molecules sufficient to induce synapse formation and several intracellular scaffolding molecules responsible for assembling active zones and recruiting synaptic vesicles. However, little is known about the molecular mechanisms coupling membrane receptors to active zone molecules during development. Using Caenorhabditis elegans, we identify an F-actin network present at nascent presynaptic terminals and required for presynaptic assembly. We unravel a sequence of events whereby specificity-determining adhesion molecules define the location of developing synapses and locally assemble F-actin. Next, the adaptor protein NAB-1 (neurabin) binds to F-actin and recruits active zone proteins SYD-1 and SYD-2 (liprin-?) by forming a tripartite complex. NAB-1 localizes transiently to synapses during development and is required for presynaptic assembly. Altogether, we identify a role for the actin cytoskeleton during presynaptic development and characterize a molecular pathway whereby NAB-1 links synaptic partner recognition to active zone assembly.

Abstract

During development, neural activity is important for forming proper connections in neural networks. The effect of activity on the gross morphology and synaptic strength of neurons has been well documented, but little is known about how activity affects different molecular components during development. Here, we examine the localization of four fluorescently-tagged presynaptic proteins, RAB-3, SNG-1/synaptogyrin, SYD-2/Liprin-?, and SAD-1/SAD kinase, in the C. elegans thermosensory neuron AFD. We show that tax-4 and tax-2, two genes that encode the cyclic nucleotide-gated channel necessary for sensory transduction in AFD, disrupt the localization of all four proteins. In wild-type animals, the synaptic vesicle (SV) markers RAB-3 and SNG-1 and the active zone markers SYD-2 and SAD-1 localize in a stereotyped, punctate pattern in the AFD axon. In tax-4 and tax-2 mutants, SV and SYD-2 puncta are more numerous and less intense. Interestingly, SAD-1 puncta are also less intense but do not increase in number. The change in puncta number can be rescued cell-autonomously in AFD. These results suggest that sensory transduction genes tax-4 and tax-2 are necessary for the proper assembly of presynapses.

Abstract

Axon-dendrite polarity is likely instructed by extrinsic cues in the developing nervous system, though the mechanisms governing this process remain to be fully elucidated. In this issue of Neuron, Shelly et al. show that the axon guidance cue Semaphorin 3A can promote dendrite growth by inhibiting axon specification.

Abstract

Neuronal polarity sets the foundation for information processing and signal transmission within neural networks. However, fundamental question of how a neuron develops and maintains structurally and functionally distinct processes, axons and dendrites, is still an unclear. The simplicity and availability of practical genetic tools makes C. elegans as an ideal model to study neuronal polarity in vivo. In recent years, new studies have identified critical polarity molecules that function at different stages of neuronal polarization in C. elegans. This review focuses on how neurons guided by extrinsic cues, break symmetry, and subsequently recruit intracellular molecules to establish and maintain axon-dendrite polarity in vivo.

Abstract

The assembly and maturation of neural circuits require a delicate balance between synapse formation and elimination. The cellular and molecular mechanisms that coordinate synaptogenesis and synapse elimination are poorly understood. In C. elegans, DD motoneurons respecify their synaptic connectivity during development by completely eliminating existing synapses and forming new synapses without changing cell morphology. Using loss- and gain-of-function genetic approaches, we demonstrate that CYY-1, a cyclin box-containing protein, drives synapse removal in this process. In addition, cyclin-dependent kinase-5 (CDK-5) facilitates new synapse formation by regulating the transport of synaptic vesicles to the sites of synaptogenesis. Furthermore, we show that coordinated activation of UNC-104/Kinesin3 and Dynein is required for patterning newly formed synapses. During the remodeling process, presynaptic components from eliminated synapses are recycled to new synapses, suggesting that signaling mechanisms and molecular motors link the deconstruction of existing synapses and the assembly of new synapses during structural synaptic plasticity.

Abstract

Nervous systems are built of a myriad of neurons connected by an even larger number of synapses. While it has been long known that neurons specifically select their synaptic partners among many possible choices during development, we only begin to understand how they make those decisions. Recent findings have started to elucidate the molecular mechanisms underlying synaptic target selection including positive as well as negative cues from synaptic partners, intermediate targets and surrounding tissues. Furthermore, emerging evidence suggests that synaptic connections are not only formed among specific sets of neurons, but also targeted to specific subcellular domains. Finally, spatial and temporal transcriptional regulation of these molecular cues represents an additional, versatile mechanism to provide wiring specificity.

Abstract

Axons navigating through the developing nervous system are instructed by external attractive and repulsive cues. Emerging evidence suggests the same cues control dendrite development, but it is not understood how they differentially instruct axons and dendrites. We studied a C. elegans motor neuron whose axon and dendrite adopt different trajectories and lengths. We found that the guidance cue UNC-6 (Netrin) is required for both axon and dendrite development. Its repulsive receptor UNC-5 repelled the axon from the ventral cell body, whereas the attractive receptor UNC-40 (DCC) was dendritically enriched and promotes antero-posterior dendritic growth. Although the endogenous ventrally secreted UNC-6 instructs axon guidance, dorsal or even membrane-tethered UNC-6 could support dendrite development. Unexpectedly, the serine-threonine kinase PAR-4 (LKB1) was selectively required for the activity of the UNC-40 pathway in dendrite outgrowth. These data suggest that axon and dendrite of one neuron interpret common environmental cues with different receptors and downstream signaling pathways.

Abstract

Many animals use their olfactory systems to learn to avoid dangers, but how neural circuits encode naive and learned olfactory preferences, and switch between those preferences, is poorly understood. Here, we map an olfactory network, from sensory input to motor output, which regulates the learned olfactory aversion of Caenorhabditis elegans for the smell of pathogenic bacteria. Naive animals prefer smells of pathogens but animals trained with pathogens lose this attraction. We find that two different neural circuits subserve these preferences, with one required for the naive preference and the other specifically for the learned preference. Calcium imaging and behavioral analysis reveal that the naive preference reflects the direct transduction of the activity of olfactory sensory neurons into motor response, whereas the learned preference involves modulations to signal transduction to downstream neurons to alter motor response. Thus, two different neural circuits regulate a behavioral switch between naive and learned olfactory preferences.

Abstract

AMPA receptors mediate most of the fast postsynaptic response at glutamatergic synapses. The abundance of AMPA receptors in neurons and at postsynaptic membranes is tightly regulated. It has been suggested that changes in synaptic AMPA receptor levels are an important regulatory event in synaptic plasticity and learning and memory. Although the local, synapse-specific regulation of AMPA receptors has been intensely studied, global, cell-wide control is less well understood. Using a forward genetic approach, we identified glutamate receptor level decreased-1 (GRLD-1), a putative RNA-binding protein that was required for efficient production of GLR-1 in the AVE interneurons in the nematode Caenorhabditis elegans. In grld-1 mutants, GLR-1 levels were markedly reduced. Consistently, glutamate-induced currents in AVE were diminished and glr-1-dependent nose-touch avoidance behavior was defective in grld-1 mutants. We propose that this evolutionarily conserved family of proteins controls the abundance of GLR-1 by regulating glr-1 transcript splicing.

Abstract

Precise formation of presynaptic structures at specific loci is critical for correctly wiring neuronal circuits. Recent findings have gradually revealed how essential cues from different sources inform the axon to define the presynaptic domain and to choose its postsynaptic target. Here, we review key molecular regulators which mediate instructive or repellent signals from multiple sources including the target cells, local guidepost cells, and distal guiding tissues.

Abstract

Presynaptic assembly requires the packaging of requisite proteins into vesicular cargoes in the cell soma, their long-distance microtubule-dependent transport down the axon, and, finally, their reconstitution into functional complexes at prespecified sites. Despite the identification of several molecules that contribute to these events, the regulatory mechanisms defining such discrete states remain elusive. We report the characterization of an Arf-like small G protein, ARL-8, required during this process. arl-8 mutants prematurely accumulate presynaptic cargoes within the proximal axon of several neuronal classes, with a corresponding failure to assemble presynapses distally. This proximal accumulation requires the activity of several molecules known to catalyze presynaptic assembly. Dynamic imaging studies reveal that arl-8 mutant vesicles exhibit an increased tendency to form immotile aggregates during transport. Together, these results suggest that arl-8 promotes a trafficking identity for presynaptic cargoes, facilitating their efficient transport by repressing premature self-association.

Abstract

Polarized trafficking of synaptic proteins to axons and dendrites is crucial to neuronal function. Through forward genetic analysis in C. elegans, we identified a cyclin (CYY-1) and a cyclin-dependent Pctaire kinase (PCT-1) necessary for targeting presynaptic components to the axon. Another cyclin-dependent kinase, CDK-5, and its activator p35, act in parallel to and partially redundantly with the CYY-1/PCT-1 pathway. Synaptic vesicles and active zone proteins mostly mislocalize to dendrites in animals defective for both PCT-1 and CDK-5 pathways. Unlike the kinesin-3 motor, unc-104/Kif1a mutant, cyy-1 cdk-5 double mutants have no reduction in anterogradely moving synaptic vesicle precursors (SVPs) as observed by dynamic imaging. Instead, the number of retrogradely moving SVPs is dramatically increased. Furthermore, this mislocalization defect is suppressed by disrupting the retrograde motor, the cytoplasmic dynein complex. Thus, PCT-1 and CDK-5 pathways direct polarized trafficking of presynaptic components by inhibiting dynein-mediated retrograde transport and setting the balance between anterograde and retrograde motors.

Abstract

A major goal of modern neuroscience research is to understand the cellular and molecular processes that control the formation, function, and remodeling of chemical synapses. In this article, we discuss the numerous studies that implicate molecules initially discovered for their functions in axon guidance as critical regulators of synapse formation and plasticity. Insights from these studies have helped elucidate basic principles of synaptogenesis, dendritic spine formation, and structural and functional synapse plasticity. In addition, they have revealed interesting dual roles for proteins and cellular mechanisms involved in both axon guidance and synaptogenesis. Much like the dual involvement of morphogens in early cell fate induction and axon guidance, many guidance-related molecules continue to play active roles in controlling the location, number, shape, and strength of neuronal synapses during development and throughout the lifetime of the organism. This article summarizes key findings that link axon guidance molecules to specific aspects of synapse formation and plasticity and discusses the emerging relationship between the molecular and cellular mechanisms that control both axon guidance and synaptogenesis.

Abstract

Synapses are specialized junctions that mediate information flow between neurons and their targets. A striking feature of the nervous system is the specificity of its synaptic connections: an individual neuron will form synapses only with a small subset of available presynaptic and postsynaptic partners. Synaptic specificity has been classically thought to arise from homophilic or heterophilic interactions between adhesive molecules acting across the synaptic cleft. Over the past decade, many new mechanisms giving rise to synaptic specificity have been identified. Synapses can be specified by secreted molecules that promote or inhibit synaptogenesis, and their source can be a neighboring guidepost cell, not just presynaptic and postsynaptic neurons. Furthermore, lineage, fate, and timing of development can also play critical roles in shaping neural circuits. Future work utilizing large-scale screens will aim to elucidate the full scope of cellular mechanisms and molecular players that can give rise to synaptic specificity.

Abstract

The assembly of specific synaptic connections during development of the nervous system represents a remarkable example of cellular recognition and differentiation. Neurons employ several different cellular signaling strategies to solve this puzzle, which successively limit unwanted interactions and reduce the number of direct recognition events that are required to result in a specific connectivity pattern. Specificity mechanisms include the action of contact-mediated and long-range signals that support or inhibit synapse formation, which can take place directly between synaptic partners or with transient partners and transient cell populations. The molecular signals that drive the synaptic differentiation process at individual synapses in the central nervous system are similarly diverse and act through multiple, parallel differentiation pathways. This molecular complexity balances the need for central circuits to be assembled with high accuracy during development while retaining plasticity for local and dynamic regulation.

Abstract

The outgrowth of axons and dendrites from neuronal cell bodies to their appropriate targets is the canonical means of creating new processes. Heiman and Shaham (2009) now show that neuronal processes can also be made by anchoring dendrite tips at their target locations while the cell body pulls away, a process termed retrograde extension.

Abstract

The wiring of the nervous system requires a complex orchestration of developmental events. Emerging evidence suggests that transient cell-cell interactions often serve as positional cues for axon guidance and synaptogenesis during the assembly of neural circuits. In contrast to the relatively stable cellular interactions between synaptic partners in mature circuits, these transient interactions involve cells that are not destined to be pre- or postsynaptic cells. Here we review the roles of these transient cell-cell interactions in a variety of developmental contexts and describe the mechanisms through which they organize neural connections.

Abstract

As fundamental units of neuronal communication, chemical synapses are composed of presynaptic and postsynaptic specializations that form at specific locations with defined shape and size. Synaptic assembly must be tightly regulated to prevent overgrowth of the synapse size and number, but the molecular mechanisms that inhibit synapse assembly are poorly understood. We identified regulator of synaptogenesis-1 (RSY-1) as an evolutionarily conserved molecule that locally antagonized presynaptic assembly. The loss of RSY-1 in Caenorhabditis elegans led to formation of extra synapses and recruitment of excessive synaptic material to presynaptic sites. RSY-1 directly interacted with and negatively regulated SYD-2/liprin-alpha, a master assembly molecule that recruits numerous synaptic components to presynaptic sites. RSY-1 also bound and regulated SYD-1, a synaptic protein required for proper functioning of SYD-2. Thus, local inhibitory mechanisms govern synapse formation.

Abstract

Wnts are secreted glycoproteins that regulate diverse aspects of development, including cell proliferation, cell fate specification and differentiation. More recently, Wnts have been shown to direct axon guidance in vertebrates, flies and worms. However, little is known about the intracellular signaling pathways downstream of Wnts in axon guidance.Here we show that the posterior C. elegans Wnt protein LIN-44 repels the axons of the adjacent D-type motor neurons by activating its receptor LIN-17/Frizzled on the neurons. Moreover, mutations in mig-5/Disheveled, gsk-3, pry-1/Axin, bar-1/beta-catenin and pop-1/TCF, also cause disrupted D-type axon pathfinding. Reduced BAR-1/beta-catenin activity in D-type axons leads to undergrowth of axons, while stabilization of BAR-1/beta-catenin in a lin-23/SCF(beta-TrCP) mutant results in an overextension phenotype.Together, our data provide evidence that Wnt-mediated axon guidance can be transduced through a beta-catenin-dependent pathway.

Abstract

Calcium/calmodulin-dependent kinase II has been suggested to produce input-specific long-term potentiation of synaptic strength. This idea has been complicated by results from Rose, Jin, and Craig demonstrating that spatiotemporally restricted NMDA receptor excitation at contiguous synapses can result in the translocation of activated CaMKII throughout the dendritic arbor.

Clathrin adaptor AP-1 complex excludes multiple postsynaptic receptors from axons in C. elegansPROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICAMargeta, M. A., Wang, G. J., Shen, K.2009; 106 (5): 1632-1637

Abstract

Neurons are highly polarized cells with morphologically and molecularly distinct axonal and dendritic compartments. It is not well understood how postsynaptic receptors are selectively enriched in dendrites in vivo. We investigated the molecular mechanisms of dendritically polarized localization of a glutamate receptor, an acetylcholine receptor, and a ROR-type receptor tyrosine kinase in the interneuron RIA in C. elegans. We found that the clathrin adaptor AP-1 complex mu1 subunit UNC-101 functions cell autonomously to maintain the correct localization of these receptors in a dynamin-dependent manner. In unc-101 mutants, instead of being dendritically enriched, all 3 receptors are evenly distributed in the axonal and dendritic compartments. Surprisingly, UNC-101 predominantly localizes to the axonal compartment, suggesting a possible transcytosis model for the dendritic targeting of neurotransmitter receptors.

The role of the ubiquitin proteasome system in synapse remodeling and neurodegenerative diseasesBIOESSAYSDing, M., Shen, K.2008; 30 (11-12): 1075-1083

Abstract

The ubiquitin proteasome system is a potent regulatory mechanism used to control protein stability in numerous cellular processes, including neural development. Many neurodegenerative diseases are featured by the accumulation of UPS-associated proteins, suggesting the UPS dysfunction may be crucial for pathogenesis. Recent experiments have highlighted the UPS as a key player during synaptic development. Here we summarize recent discoveries centered on the role of the UPS in synapse remodeling and draw attention to the potential link between the synaptic UPS dysfunction and the pathology of neurodegenerative diseases.

Abstract

Polarity is an essential feature of many cell types, including neurons that receive information from local inputs within their dendrites and propagate nerve impulses to distant targets through a single axon. It is generally believed that intrinsic structural differences between axons and dendrites dictate the polarized localization of axonal and dendritic proteins. However, whether extracellular cues also instruct this process in vivo has not been explored. Here we show that the axon guidance cue UNC-6/netrin and its receptor UNC-5 act throughout development to exclude synaptic vesicle and active zone proteins from the dendrite of the Caenorhabditis elegans motor neuron DA9, which is proximal to a source of UNC-6/netrin. In unc-6/netrin and unc-5 loss-of-function mutants, presynaptic components mislocalize to the DA9 dendrite. In addition, ectopically expressed UNC-6/netrin, acting through UNC-5, is sufficient to exclude endogenous synapses from adjacent subcellular domains within the DA9 axon. Furthermore, this anti-synaptogenic activity is interchangeable with that of LIN-44/Wnt despite being transduced through different receptors, suggesting that extracellular cues such as netrin and Wnts not only guide axon navigation but also regulate the polarized accumulation of presynaptic components through local exclusion.

Abstract

Cell adhesion molecules of the Immunoglobulin superfamily (IgCAMs) play diverse functions during neural development. Previously, we have identified SYG-1/Neph1 and SYG-2/Nephrin, IgCAMs necessary for synaptic specificity in Caenorhabditis elegans. Here, we conduct an in vivo structure-function analysis of SYG-1 and SYG-2 to identify domains of SYG-1 and SYG-2 necessary for heterophilic binding as well as synaptic specificity. We find the first Ig domain of SYG-1 and the first 5 Ig domains of SYG-2 are necessary and sufficient for their binding in vivo, as well as for synapse formation. We also find the SYG-2 cytoplasmic domain is required for SYG-2 subcellular trafficking, while the intracellular region of SYG-1 is required for synaptic function at earlier developmental stages, but is dispensable for later stages. This study defines the domain requirements for SYG-1/SYG-2 heterophilic binding and suggests that unknown SYG-1 extracellular interactors may play a role in SYG-1-mediated synaptic specificity.

Abstract

Synapses are individually operated, computational units for neural communication. To manipulate physically individual synapses in a living organism, we have developed a laser ablation technique for removing single synapses in live neurons in C. elegans that operates without apparent damage to the axon. As a complementary technique, we applied microfluidic immobilization of C. elegans to facilitate long-term fluorescence imaging and observation of neuronal development. With this technique, we directly demonstrated the existence of competition between developing synapses in the HSNL motor neuron.

Abstract

The identification of synaptic partners is challenging in dense nerve bundles, where many processes occupy regions beneath the resolution of conventional light microscopy. To address this difficulty, we have developed GRASP, a system to label membrane contacts and synapses between two cells in living animals. Two complementary fragments of GFP are expressed on different cells, tethered to extracellular domains of transmembrane carrier proteins. When the complementary GFP fragments are fused to ubiquitous transmembrane proteins, GFP fluorescence appears uniformly along membrane contacts between the two cells. When one or both GFP fragments are fused to synaptic transmembrane proteins, GFP fluorescence is tightly localized to synapses. GRASP marks known synaptic contacts in C. elegans, correctly identifies changes in mutants with altered synaptic specificity, and can uncover new information about synaptic locations as confirmed by electron microscopy. GRASP may prove particularly useful for defining connectivity in complex nervous systems.

Abstract

Synapses are specialized sites of cell contact that mediate information flow between neurons and their targets. Genetic screens in the nematode C. elegans have led to the discovery of a number of molecules required for synapse patterning and assembly. Recent studies have demonstrated the importance of guidepost cells in the positioning of presynaptic sites at specific locations along the axon. Interestingly, these guideposts can promote or inhibit synapse formation, and do so by utilizing transmembrane adhesion molecules or secreted factors that act over relatively larger distances. Once the decision of where to build a presynaptic terminal has been made, key molecules are recruited to assemble synaptic vesicles and active zone proteins at that site. Multiple steps of this process are regulated by ubiquitin ligase complexes. Interestingly, some of the molecules involved in presynaptic assembly also play roles in regulating axon polarity and outgrowth, suggesting that different neurodevelopmental processes are molecularly integrated.

Abstract

Neural circuits are assembled through the coordinated innervation of pre- and postsynaptic partners. We show that connectivity between two interneurons, AIY and RIA, in Caenorhabditis elegans is orchestrated by a pair of glial cells that express UNC-6 (netrin). In the postsynaptic neuron RIA, the netrin receptor UNC-40 (DCC, deleted in colorectal cancer) plays a conventional guidance role, directing outgrowth of the RIA process ventrally toward the glia. In the presynaptic neuron AIY, UNC-40 (DCC) plays an unexpected and previously uncharacterized role: It cell-autonomously promotes assembly of presynaptic terminals in the immediate vicinity of the glial cell endfeet. These results indicate that netrin can be used both for guidance and local synaptogenesis and suggest that glial cells can function as guideposts during the assembly of neural circuits in vivo.

Abstract

Nervous system function is mediated by a precisely patterned network of synaptic connections. While several cell-adhesion and secreted molecules promote the assembly of synapses, the contribution of signals that negatively regulate synaptogenesis is not well understood. We examined synapse formation in the Caenorhabditis elegans motor neuron DA9, whose presynapses are restricted to a specific segment of its axon. We report that the Wnt lin-44 localizes the Wnt receptor lin-17/Frizzled (Fz) to a subdomain of the DA9 axon that is devoid of presynaptic specializations. When this signaling pathway, composed of the Wnts lin-44 and egl-20, lin-17/Frizzled and dsh-1/Dishevelled, is compromised, synapses develop ectopically in this subdomain. Conversely, overexpression of LIN-44 in cells adjacent to DA9 is sufficient to expand LIN-17 localization within the DA9 axon, thereby inhibiting presynaptic assembly. These results suggest that morphogenetic signals can spatially regulate the patterning of synaptic connections by subdividing an axon into discrete domains.

Abstract

The presynaptic regions of axons accumulate synaptic vesicles, active zone proteins and periactive zone proteins. However, the rules for orderly recruitment of presynaptic components are not well understood. We systematically examined molecular mechanisms of presynaptic development in egg-laying synapses of Caenorhabditis elegans, demonstrating that two scaffolding molecules, SYD-1 and SYD-2, have key roles in presynaptic assembly. SYD-2 (liprin-alpha) was previously shown to regulate the size and the shape of active zones. We now show that in syd-1 and syd-2 mutants, synaptic vesicles and numerous other presynaptic proteins fail to accumulate at presynaptic sites. SYD-1 and SYD-2 function cell-autonomously at presynaptic terminals, downstream of synaptic specificity molecule SYG-1. SYD-1 is likely to act upstream of SYD-2 to positively regulate its synaptic assembly activity. These data imply a hierarchical organization of presynaptic assembly, in which transmembrane specificity molecules initiate synaptogenesis by recruiting a few key scaffolding proteins, which in turn assemble other presynaptic components.

Abstract

Synapse formation is initiated by cell-cell contact between appropriate pre- and postsynaptic cells and is followed by recruitment of protein complexes in both pre- and postsynaptic compartments. In this issue of Neuron, Lyles et al. show that in cultured Aplysia neurons, clustering of an mRNA at nascent synapses is not only induced by the recognition between synaptic partners, but is also required for further synaptic development and maintenance.

Abstract

Neurons are connected with a high degree of specificity in neuronal circuits. Axon guidance mechanisms are responsible for directing axons to their approximate target region. It is not well understood how precise synaptic connections form between specific pre- and postsynaptic neurons within the target area. Recent analysis of a group of cell surface proteins in different systems has shed light on the diverse cellular and molecular mechanisms that generate the precise patterns of connectivity.

Abstract

Synaptic plasticity is thought to be a key process for learning, memory and other cognitive functions of the nervous system. The initial events of plasticity require the conversion of brief electrical signals into alterations of the biochemical properties of synapses that last for much longer than the initial stimuli. Here we show that a regulator of synaptic plasticity, calcium/calmodulin-dependent protein kinase IIalpha (CaMKII), sequentially translocates to postsynaptic sites, undergoes autophosphorylation and gets trapped for several minutes until its dissociation is induced by secondary autophosphorylation and phosphatase 1 action. Once dissociated, CaMKII shows facilitated translocation for several minutes. This suggests that trapping of CaMKII by its targets and priming of CaMKII translocation may function as biochemical memory mechanisms that change the signaling capacity of synapses.

In and out of the postsynaptic region: signalling proteins on the moveTRENDS IN CELL BIOLOGYMeyer, T., Shen, K.2000; 10 (6): 238-244

Abstract

Reversible translocation of signalling proteins to and from their sites of action has emerged as an important theme in signal transduction. The recent findings of the stimulus-induced translocation of Ca2+-calmodulin-dependent protein kinase II (CaMKII) and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor to and from the postsynaptic region are model cases for understanding how the dynamic localization of signalling proteins is used to regulate signal transduction.

Abstract

Binding interactions between the plasma membrane and the cytoskeleton define cell functions such as cell shape, formation of cell processes, cell movement, and endocytosis. Here we use optical tweezers tether force measurements and show that plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2) acts as a second messenger that regulates the adhesion energy between the cytoskeleton and the plasma membrane. Receptor stimuli that hydrolyze PIP2 lowered adhesion energy, a process that could be mimicked by expressing PH domains that sequester PIP2 or by targeting a 5'-PIP2-phosphatase to the plasma membrane to selectively lower plasma membrane PIP2 concentration. Our study suggests that plasma membrane PIP2 controls dynamic membrane functions and cell shape by locally increasing and decreasing the adhesion between the actin-based cortical cytoskeleton and the plasma membrane.

Abstract

Calcium-calmodulin-dependent protein kinase II (CaMKII) is thought to increase synaptic strength by phosphorylating postsynaptic density (PSD) ion channels and signaling proteins. It is shown that N-methyl-D-aspartate (NMDA) receptor stimulation reversibly translocates green fluorescent protein-tagged CaMKII from an F-actin-bound to a PSD-bound state. The translocation time was controlled by the ratio of expressed beta-CaMKII to alpha-CaMKII isoforms. Although F-actin dissociation into the cytosol required autophosphorylation of or calcium-calmodulin binding to beta-CaMKII, PSD translocation required binding of calcium-calmodulin to either the alpha- or beta-CaMKII subunits. Autophosphorylation of CaMKII indirectly prolongs its PSD localization by increasing the calmodulin-binding affinity.

Abstract

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a serine/threonine protein kinase that regulates long-term potentiation and other forms of neuronal plasticity. Functional differences between the neuronal CaMKIIalpha and CaMKIIbeta isoforms are not yet known. Here, we use green fluorescent protein-tagged (GFP-tagged) CaMKII isoforms and show that CaMKIIbeta is bound to F-actin in dendritic spines and cell cortex while CaMKIIalpha is largely a cytosolic enzyme. When expressed together, the two isoforms form large heterooligomers, and a small fraction of CaMKIIbeta is sufficient to dock the predominant CaMKIIalpha to the actin cytoskeleton. Thus, CaMKIIbeta functions as a targeting module that localizes a much larger number of CaMKIIalpha isozymes to synaptic and cytoskeletal sites of action.